1. Field of the Invention
The present invention relates to methods of reducing the precipitation of metal sulfides from acid solutions when acid treating a sour well or the like.
2. Description of the Prior Art
Acid solutions are commonly used to treat oil and/or gas wells. For example, subterranean well formations are often contacted with aqueous acid solutions to increase the permeabilities of the formations whereby the production of oil and/or gas therefrom is increased. Acid solutions are also employed to clean out perforations and tubular goods in wells as well as to clean piping and equipment.
When the well being treated is sour, i.e., the fluids produced contain a significant quantity of sulfide compounds, and particularly hydrogen sulfide, problems are often encountered as a result of the precipitation of iron sulfide from the acid treating solution. That is, dissolved sulfide ions and dissolved ferrous ions from ferrous sulfide scale and/or the subterranean formation being treated react to form ferrous sulfide in the acid solution. The ferrous sulfide precipitates from the acid solution when the pH of the solution reaches a level greater than about 1.9. The pH value of spent acid solution used to treat subterranean well formations is greater than 1.9, and therefore, ferrous sulfide dissolved in an unspent acid solution precipitates at some point before becoming spent. The ferrous sulfide precipitate can plug the treated formation and cause serious damage to the well.
Attempts have been made to reduce the precipitation of ferrous sulfide by maintaining the pH of the acid solution below 1.9. However, such attempts have generally failed as a result of the pH rising to above 1.9 at some point in the acidizing procedure.
While iron-containing scale dissolved in an acid solution can be in the ferric [Fe (III)] oxidation state in producing wells whereby ferric hydroxide (FeOH.sub.3) precipitates, such ferric scale is generally not present in hydrogen sulfide producing wells. That is, a well which contains hydrogen sulfide does not contain exposed ferric scale because the hydrogen sulfide rapidly converts such scale to ferrous sulfide scale.
A method and composition for preventing the precipitation of ferrous sulfide and sulfur when acidizing a sour well are disclosed in U.S. Pat. No. 4,633,949 issued on Jan. 6, 1987. In accordance with that disclosure, an iron complexing agent such as ethylenediaminetetraacetic acid (EDTA) and an iron reducing agent such as erythorbic acid are added to the acidizing solution used. The iron complexing agent is intended to stabilize ferrous ion in solution in order to inhibit precipitation of ferrous sulfide, and the iron reducing agent is intended to reduce ferric ion to ferrous ion in order to inhibit the formation of elemental sulfur. Thus, the invention disclosed attempts to control precipitation of both elemental sulfur and ferrous sulfide by complexing ferrous ion and reducing ferric ion to ferrous ion.
U.S. Pat. No. 4,888,121 issued on Dec. 19, 1989 also discloses compositions and methods for controlling precipitation when acidizing sour wells. In accordance with that disclosure, an iron sequestering agent such as an aminopolycarboxylic acid is added to the acid treating solution to sequester dissolved iron therein and a sulfide modifier such as acetal is added to combine with dissolved sulfides. The combination of iron sequestering agent and sulfide modifier functions to inhibit or reduce the precipitation of ferric hydroxide, ferrous sulfide and free sulfur from the acid solution.
While the prior art methods have achieved varying degrees of success in preventing precipitation of metal sulfides from acid treating solutions, there is still a need for improved sour well acidizing methods and compositions which are relatively inexpensive to perform and use and produce little or no precipitate.